Instrument manifolds are commonly employed in differential pressure systems between the source of the differential pressure and the pressure transducer, monitor or meter. In a typical installation, a three-valve or five-valve instrument manifold is installed between an orifice plate assembly and a differential pressure transducer to normally transmit a pair of pressure signals to the transducer, and to allow for intermittent testing of the measuring system while line fluid passes through the orifice plate assembly. The instrument manifold may be connected to the orifice plate assembly by either remote couplings or direct (close) couplings. While the remote coupling technique provides a high degree of flexibility with respect to placement of the instrument manifold, direct or close coupling of the orifice plate assembly and the instrument manifold is often preferred to reduce travel of the pressure signals and thus increase system accuracy, to reduce fluid-tight interconnections and thereby increase pressure signal reliability, to simplify rod-out operations, and to significantly reduce instrument manifold installation costs.
Instrument manifold installation costs can most dramatically be reduced when the close coupling equipment independently provides the structural connection between the orifice plate assembly and the instrument manifold. While the savings from this installation technique are significant, the use of direct or close coupling between an orifice plate assembly and an instrument manifold has long been limited, primarily due to justified concern that over an extended period of time the close coupling interconnection may be unable to withstand the industrial environment in which these components are placed. In many cases, this concern is due to the periodic or continuous presence of high vibration of the fluid line in which the orifice plate assembly is placed. The concern of the system operator is that vibration of the orifice plate assembly will be transmitted through the close coupling connectors to the instrument manifold, resulting in leakage and/or structural failure of the close coupling connectors. This problem is not easily overcome, since any mechanism which either reduces this vibration or is adapted to withstand this vibration over a long period of time must be cost effective and easy to install, preferably does not increase the relatively short distance between the orifice plate assembly and the instrument manifold, and ideally is highly versatile so that it can be employed between various types of differential pressure assemblies and instrument manifolds.
Many types of coupling devices are not suitable for transmitting pressure signals from an orifice plate assembly to an instrument manifold. U.S. Pat. No. 2,852,281, for example, discloses a fluid pressure coupling that uses a wedged sealing sleeve. Orifice plate assemblies are conventionally provided with tapered NPT threads which form a fluid-tight metal-to-metal seal between the orifice plate assembly body and the threaded coupling, and accordingly a coupling with straight threads and an O-ring seal is not practically usable with conventional orifice plate assemblies. An end portion of each coupling adjacent the instrument manifold must also be rotatable, so that apertures within that end portion can be rotatably aligned with corresponding apertures in the instrument manifold flange for structurally interconnecting each coupling with the instrument manifold. U.S. Pat. Nos. 2,343,325, 2,919,147, and 3,151,893 each disclose couplings which are intended to enable one end of a coupling to be rotated at a selected angular position while of these couplings, however, again discloses straight threads rather than tapered NPT threads of the type used in a conventional orifice plate assembly. Moreover, the couplings do not include a pair of apertures or other suitable means for mechanically connecting the free end of the coupling with a flange of an instrument manifold. Finally and most importantly, these patents do not teach a practical solution to the problem which has reduced the commercial use of direct couplings between an orifice plate assembly and an instrument manifold, namely to provide a mechanism which can withstand the previously described vibrational forces over a long period of time, and thereby overcome the leakage concern.
Couplings which structurally interconnect an orifice plate assembly with an instrument manifold thus must satisfy special problems inherent in this application, and generally are specially adapted for this particular use. Direct coupling of an orifice plate assembly with an instrument manifold is conventionally accomplished by a pair of nipples each having NPT threads at each end, with a "football" mechanism including a pair of through apertures provided at the instrument manifold end of each nipple for mechanically interconnecting the coupling to the instrument manifold. U.S. Pat. No. 4,672,728 discloses a pair of nipples for structurally interconnecting an instrument manifold with an orifice plate assembly and for passing the pair of signals from the orifice plate assembly through the instrument manifold and to a pressure transmitter. This patent also discloses a preferred instrument manifold having a removable flange connected to the manifold body with a pair of specially adapted fittings for forming a fluid-tight connection between each football and the corresponding fitting. Apertures in each of the footballs are aligned with corresponding apertures in the instrument manifold flange, so that the instrument manifold can be structurally connected to the orifice plate assembly by the pair of nipples and footballs. Other types of direct or close couplings between an instrument manifold and an orifice plate assembly are shown in U.S. Pat. No. 4,672,728, which also depicts the previously described NPT ports formed in a pair of circular flange bodies of an orifice plate assembly.
The direct coupling connectors disclosed in the latter two patents each serve the function of sealing an NPT port in an orifice plate assembly with a corresponding port in an instrument manifold, and also provide means for independently structurally interconnecting the instrument manifold with the orifice plate assembly. While these connectors have been widely used, they often do not satisfy the customer's reliability concern for high vibration applications. When placed in such an environment, the vibrating connectors, coupled with the weight of the instrument manifold and often the weight of the pressure transducer, may cause the nipples to loosen, thereby resulting in leakage of pressure and thus poor signal reliability. These connectors are also particularly susceptible to leakage, or even structural failure, when the instrument manifold (or transmitter connected thereto) is subjected to a vector force perpendicular to a plane passing through each of the axes of the connectors. Since the connectors are spaced apart, they are capable of withstanding a reasonable vector force within this plane, but are not able to withstand a similar magnitude vector force perpendicular to this plane. Accordingly, customers faced with such a high vibration environment, or faced with other environments which would cause one to question whether the nipples can continually withstand the forces which may act on the instrument manifold without allowing signal pressure leakage, often utilize the much more expensive and less desirable installation technique of providing a separate "platform" for mounting the instrument manifold structurally separate from the orifice plate assembly, and then interconnect the orifice plate assembly and the remote instrument manifold with flexible fluid lines.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter provided for reliably forming a structural interconnection between a standard instrument manifold and an orifice plate assembly using a pair of connector flange assemblies as described herein.